The two services had also already established a shared command—the Mobile Riverine Force—to tackle insurgents in the Mekong region. The Army was impressed by the sailing branch’s Patrol Air Cushion Vehicles after seeing them in action.

A U.S. Navy Patrol Air Cushion Vehicle exits the USS Gunston Hall in Vietnam. Navy photo

The vehicles used their cushion of air to ride across the rivers and canals and flooded rice paddies and muddy lowlands. The hovercrafts were also fast and could move from the water to land at relatively high speeds.

In contrast, Army armored formations regularly got bogged down in the Mekong. The ground combat branch relied heavily on helicopters and Navy boats to schlep troops around.

The air cushion vehicles offered a potential solution. The vehicles were like combinations of tanks, helicopters and amphibious landing craft.

The connection to helicopters and the new “air cavalry” was most obvious. Bell made the Army’s Huey helicopters in addition to the SK-5s.

The Army planned to arm each of these craft with machine guns—including a rapid-firing Minigun—and automatic grenade launchers. The grenade launchers were originally designed for gunships.

In the end, just one vehicle got the grenade launcher. The Army removed the machine guns from the main cabin of another in order to make more space for troops and cargo.

The ACV Unit—also known as the Air Cushion Vehicle Test Unit, Armor Platoon Air Cushioned and 39th Cavalry Platoon—spent most of its time patrolling the Mekong and participating in attacks on suspected base camps.

The vehicles also inserted commandos into enemy territory and ambushed enemy fighters. The hovercraft had shock value because they were so loud. Guerrillas reportedly called all American hovercraft “monsters.”

But the craft made no sound when their engines were off—and each of them had radar. Able to get into remote areas, the hovercraft could power down and act as sort of impromptu guard posts.

The initial reports on the platoon were positive. The U.S. Army Combat Developments Command quickly recommended buying more hovercraft to form a larger contingent.

U.S. Army SK-5 hovercrafts in Vietnam. Army photos

The proposed Air Cushion Vehicle Assault Troop would have had 12 vehicles, including two SK-6s. This larger variant would have carried extra radios or other equipment.

This planned hovercraft cavalry unit would also have had heavier weapons. Army evaluators suggested adding cannons, recoilless rifles, rockets and anti-tank missiles to give the vehicles more firepower.

The Army’s leadership in Vietnam disagreed and said that USACDC’s proposal was “premature,” according to an official report. The service purchased no additional vehicles. The SK-6 design languished.

The hovercraft did have significant problems. For one, the soldiers needed training to repair the specialized vehicles when they broke. Hovercraft “pilots” also required extensive instruction.

Troops had to learn to safely ride in and on the new vehicles, as well. One soldier died during a mission when he accidentally fell into the craft’s large lift fan.

The ground combat branch also stipulated that the vehicles work in pairs to be able to protect each other in a fight. A single broken hovercraft was fine if you had three on hand.

But on Jan. 9, 1970, one was destroyed. From that point on, operations ground to a halt if one of the remaining vehicles was out of commission.

In August, the platoon stopped functioning completely after a mine blew up another hovercraft. The ACV Unit’s moment had passed.

The war in Vietnam was winding down and the Army had lost interest in the concept. The last SK-5 became a museum piece.

After leaving Southeast Asia, the Army and Navy both gave up the idea of using hovercrafts. Now, the Navy’s air cushion landing craft are the only such vehicles left in the American military.